1. Field of the Invention
This invention relates generally to the field of stress measurement and more particularly to a novel holograpic instrument for determining the stress in the wall of a borehole.
2. Prior Art
An important, and in many cases crucial, requirement in many geological activities is the determination or measurement of the in-situ stress field, including both stress magnitude and stress direction, in geological structures. Some of these geological activities are
Utilization of geothermal energy; PA1 Tunnel excavation; PA1 Discrimination of teleseismic signals to determine their origin (earthquake or explosion); and PA1 Earthquake prediction. PA1 Aggson, J. R., Bureau of Mines Borehole Deformation Gage for Determining In-Situ Stress, Bureau of Mines Information Circular 8585, July 25, 1972. PA1 Pallister, G. F., N. C. Gay and N. G. W. Cook, Measurement of the Virgin State of Stress in rock at Depth, Proc. of the Second Congress, International Society of Rock Mechanics, Belgrade, 25, 1970. PA1 Merrill, R. H., In-Situ Determination of Stresses by Relief Techniques, State of Stress in Earth's Crust, W. R. Judd, ed. 343, Elsevier Co., 1964.
Utilization of geothermal energy, for example, involves drilling of boreholes or passages into subterranean rock formations which are exposed to geothermal heating and circulation of water through such passages for heating of the water by heat transfer from the rock. Efficient heat transfer from the rock to the water requires fracturing of the rock walls of the water passages. Efficient and economical fracturing of the rock, in turn, necessitates a precise knowledge of the principal stresses in the rock.
Knowledge of the basic data related to rock properties is also required in tunnel excavation. Among the required data is information pertaining to the in-situ stress field in the rock. Such information is required to both determine the stability of the tunnel wall and insure the safety of tunnel occupants. As tunneling proceeds to greater depth, in-situ stress field measurements at operational depths become important to production planning. The risk of rock bursts and cave-ins in a tunnel is, in general, dependent on the stability of the surrounding stress field (tectonic and overburden) and its longterm response to the presence of the tunnels. Thus, one primary requirement, in addition to knowledge of the underground geology, in the safe design of tunnels, is the determination of both the magnitude and direction of the in-situ stress fields.
A variety of in-situ stress measurement techniques and devices have been devised. Examples of such techniques and devices are described in the following publications:
These and other previously available in-situ stress measurement techniques, however, do not provide a complete measure of the state of stress in the vicinity of the borehole, i.e. tunnel or vertical bore. For example, the Bureau of Mines borehole deformation gage, which is currently the best available technique, provides sufficient information to calculate the state of stress in a plane normal to the borehole. In order to obtain a measure of all the components of stress, borehole deformation measurements would be required from three non-parallel boreholes. In calculating the stability of openings, it is also necessary to have a measure of the in-situ stress (tectonic plus overburden) parallel to the axis of the vertical borehole. Disadvantages of the Bureau of Mines borehole deformation gage are that the state of stress at a point is difficult to measure even using three holes and additional expenses are incurred in drilling three holes rather than one. Even after three holes are drilled, the technique provides a measure of three-dimensional average ground stress components because of the nature of the instrumentation used. If this technique were utilized to obtain a measure of the in-situ stresses as a function of depth, the expense incurred would be large, especially if the application is for the design of a deep mine. It should be pointed out however, that the U.S. Bureau of Mines gage is a working instrument.
The earlier mentioned copending application Ser. No. 530,626, discloses an improved technique for determining stress in load-bearing structures, both man-made and geological structures. Simply stated, the technique involves recording successively on the same holographic recording medium two holograms of a selected surface region of the load-bearing structure and, in the interval between the two recordings, forming a stress relief in the structure in stress relieving relation to the selected surface region. The resulting composite hologram recorded on the medium is a double exposure hologram containing holographic information which represents the displacement of the surface. Both stress direction and stress magnitude in the structure may be derived from these date, using independent knowledge of the elastic constants of the structure medium. The holographic information contained in the double exposure hologram is a fringe pattern resulting from the deformations or displacements which occur at the surface of the structure between the first and second holograms by virtue of the stress relief formed in the structure. According to the preferred practice of the invention, the stress relief is formed by drilling a hole into the structure within its selected holographically imaged surface region.